The binding of alpha-chymotrypsin to black-eyed pea trypsin/chymotrypsin inhibitor (BTCI) has been studied using the inhibitory activity against the enzyme and the formation of the complex enzyme/inhibitor followed by measurements of fluorescence polarization. Apparent equilibrium constants were estimated for several temperatures and the values obtained range from 0.32 x 10(7) to 1.36 x 10(7) M(-1). The following values were found from van't Hoff plots: delta H(0)vh = 10.8 kcal mol(-1) (from inhibitory assays) and 11.1 kcal mol(-1) (from fluorescence polarization); delta S(0) = 67.9 and = 67.8 kcal K(-1) mol(-1), respectively. Calorimetric binding enthalpy was determined (corrected for the ionization heat of the buffer) and the resulting value was delta H(0)cal = 4.9 kcal mol(-1). These results indicate that the binding of chymotrypsin to BTCI is an entropically driven process.